REPRODUCTION AND DEVELOPMENT 281 



without function. For example, the vermiform appendix of Man is 

 a small blind tube continuous with the ca!cum, a pouch at the junc- 

 tion of the large and small intestine, and is without function in 

 the digestive process. Such structures are called vestigial organs, 

 for they appear to be vestiges of functional structures of lower ani- 

 mals. In the rat and the rabbit there is no vermiform appendix, the 

 CcTcum being long and including at its end the portion that is 

 homologous with the appendix of Man. 



Similarly, some of the structures that function in one fashion in 

 lower animals are partially obliterated during the development of 

 higher forms and modified to assume quite different functions. 

 For example, the visceral pouches in the aquatic vertebrates become 

 gills and are the chief respiratory organ; in mammals these pouches 

 appear in the embryo, but some of them are obliterated while 

 others are transformed as organs of quite different function, none 

 of them being concerned in the development of the respiratory or- 

 gans. The appearance during development of a considerable num- 

 ber of such vestiges has led to the formulation of a biogenetic law, 

 which states that the development of an individual repeats the 

 history of the species as derived from ancestral types. 



Without going into the great mass of facts of anatomy and of 

 development that agree with the principle of biogenesis, or recapit- 

 ulation, and the facts that appear to make it of limited application, 

 it may be stated with accuracy that the occurrence of vestigial 

 structures and the conversion of structures to new uses are most 

 rationally explained by this law. However, the application of the 

 law must be made only in the most general way, for in no case is 

 any developing stage of an embryo identical with the adult of a 

 lower species. There is frequently a resemblance which is highly 

 suggestive that in the development of the individual we are wit- 

 nessing a sort of historical panorama, but there is no absolute 

 identity of higher embryo and lower adult. Human embryos de- 

 velop endodermal pouches which are similar in position and struc- 



